Myocardial ischemia is a common condition with life-threatening consequences. Myocardial ischemia occurs when blood flow to the heart muscle (myocardium) is obstructed by a partial or complete blockage of a coronary artery. Severe myocardial ischemia may lead to myocardial infarction. Myocardial ischemia may also cause a serious abnormal heart rhythm (arrhythmia), which can cause fainting or even sudden death. The prevalence of ischemic heart disease is estimated to be approximately 6.40% in the U.S. population.
Because of the high prevalence of ischemic heart disease and the very serious nature of its possible effects, monitoring and quick diagnosis of myocardial ischemia are of great importance to clinicians. This is particularly true of patients with implantable cardiac rhythm management (CRM) devices because the prevalence of ischemic heart disease is relatively high in the population of patients with such devices.
Electrocardiography is an important tool for the monitoring and diagnosis of various cardiac conditions. Extracellular cardiac electrical fields are generated by ion fluxes across cell membranes and between adjacent cells. These ion currents are synchronized by cardiac activation and recovery sequences to generate a cardiac electrical field in and around the heart that varies with time during cardiac cycles. The process of measuring these time varying cardiac electrical fields is known as electrocardiography. Electrocardiography represents a widely used clinical tool for the detection and diagnosis of a broad range of cardiac conditions.
Acute myocardial ischemia can be observed using electrocardiography. Severe acute ischemia can reduce the resting membrane potential, shorten the duration of the action potential in the ischemic area, and decrease the rate of rise and amplitude of phase 0 of the action potential (when voltage-gated sodium channels open). These changes cause a voltage gradient between normal and ischemic zones that lead to current flow between these regions, represented on an electrocardiogram (ECG) by deviation of the ST segment. As such, an acute ischemic condition, such as acute myocardial infarction, is recognized by electrocardiography through ST-segment elevation.